1. Field of the Invention
This invention relates to a device useful in enhancing gray scale images on various types of displays. More particularly, the invention provides a device which enables a computer using a VGA board to produce 7-bit (or higher) gray scale images on either a VGA monitor or gray scale monitor.
2. Description of the Art
Digital computer and monitor systems are now an integral part of the medical imaging process. These systems allow analysts to examine medical images in real time to quickly analyze clinical indications for purposes of diagnosis.
Because the diagnostic usefulness of a medical image is directly tied to the quality of the image, it is desirable to preserve as much image detail as possible. In many medical applications, gray scale images (i.e. images formed of shades of gray) are used as the principle diagnostic tool. When observing a colored image, the human brain may have difficulty perceiving color change as gradual, seamless change; instead, the brain sees color that starts and stops, as if there were a sudden characteristic change in tissue being observed. On the other hand, gray scale images readily show shading, crevices, cracks, indentations and other tissue characteristics in a realistic manner which is easy for the human eye to detect. Thus, much of the medical imaging industry has concentrated on producing gray scale images with enhanced detail as opposed to enhanced colored images.
In order to produce detailed gray scale images, the industry has developed customized imaging boards to drive various types (i.e. there is no widely recognized hardware standard for gray scale monitors) of gray scale monitors. As a different imaging board must be custom built to drive each different type of gray scale monitor, these boards can be prohibitively expensive for many applications. In addition, as these custom boards comprise specific hardware, these specialized gray scale systems require software to be written specifically for the hardware used thus extending development time and cost and limiting flexibility.
Unlike the gray scale monitors for which there is no monitor standard, the colored Vector Graphics Adapter (VGA) monitor has become the widely recognized standard for color monitors in the computer industry. The VGA monitor is now included as a standard piece of hardware with most personal computers. Attempting to take advantage of the abundance of VGA monitors to avoid the high costs associated with the specialized gray scale systems, the imaging industry has developed software that can be used with VGA monitors to produce gray scale images. While a VGA monitor and associated software is less expensive than the specialized gray scale systems, the quality of a gray scale image on a VGA monitor is inferior.
VGA graphics boards drive what are commonly known as raster type monitors wherein graphic images are created by displaying a large number of tiny dots, or pixels, having various brightness levels on the face of a monitor. An electron beam is magnetically deflected to excite each pixel on the monitor.
VGA monitors are full color monitors wherein each pixel is a triad of phosphorous dots in the three primary colors--red, green and blue. Three 6-bit digital data bytes, one byte corresponding to each dot, are stored in a computer memory and contain instructions for exciting each of the three dots. The data bytes are transferred to a digital to analog converter on a conventional computer display board to generate three separate analog video signals. The three analog video signals control the intensities of three electron beams, each beam directed at a different phosphorous dot.
As a 6-bit word defines the intensity of each electron beam, each electron beam can have a total combination of 2.sup.6 or 64 different intensity levels. To produce a black, white or gray pixel on a VGA monitor, the intensity of all three primary channels (red, green and blue) must be equal. Thus, a standard VGA system can generate gray scale images having only 64 unique gray shades. On the other hand, many of the specialized gray scale systems can produce 2.sup.8, 2.sup.10 or as many as 2.sup.12 different gray levels.
Often, subtle clinical information in a medical image is contained in image areas of only slightly different intensity. Due to the limitation on the gray scale image they can display, VGA images with only 64 different gray levels tend to look like topographic contour maps rather than being smoothly shaded across the gray scale. In many cases, inferior VGA images lack detail which is needed or would be useful for proper diagnostic purposes.
Therefore, it can be seen that there is a need for an inexpensive system to enable a relatively inexpensive monitor to be used to produce a high quality gray scale image with many different gray levels.